In building construction, interior walls are generally formed by attaching preformed sheets of gypsum wallboard to a supporting frame. These sheets of wallboard are susceptible to damage, especially at outside corners. In order to protect an outside corner of a drywall structure, a cornerbead is often installed on the corner prior to painting. A conventional cornerbead is typically a thin, elongate strip of metal or plastic with a right angle bend along the strip's longitudinal axis. The right angle bend forms a nose and two flanges are joined to respective opposing sides of the nose at the flanges' respective inner edges. Conventionally, each flange is 1¼ inch wide. Holes may be provided in the flanges for nailing or screwing the cornerbead to the underlying drywall structure.
During the construction process an installer must place the cornerbead on the desired outside corner, hold the cornerbead in place with one hand and attach the cornerbead to the drywall using nails or screws with the other hand. Joint compound is then applied over the flange and the adjacent wallboard to provide a smooth surface transition for painting. This process is very time consuming and must be repeated many times in a typical new construction project.
A conventional form of cornerbead that avoids some of the difficulties described above is made of synthetic plastic material and has a coating of water-activated adhesive on the interior surfaces of the flanges. During installation, the installer wets the interior surfaces of the flanges and then places the cornerbead against the outside corner. The adhesive allows repositioning of the cornerbead during a short interval before the adhesive dries and bonds the cornerbead to the drywall. When the adhesive has dried, the installer applies joint compound to provide a smooth surface for painting. A practical disadvantage of this type of cornerbead becomes apparent in the event of an unexpected rain shower while the installer is transporting a load of cornerbead to a job site on the open bed of a pick-up truck: the adhesive is activated and handling of the cornerbead then becomes difficult.
The outer edges of the flanges of conventional cornerbead are straight. In the event that nose of the cornerbead is struck after the joint compound has hardened, the impact may cause the flange to move relative to the underlying drywall structure, which may stress the joint compound to such an extent as to create a crack that can propagate a foot or more along the straight outer edge of the flange.
It has previously been proposed that a cornerbead made of synthetic plastic material should be provided with a strip of fiberglass mesh tape on each flange. The mesh tape adheres to the flanges and, when the cornerbead is placed against the drywall corner, adheres to the drywall in a manner that allows repositioning for an extended period of time. The mesh tape, being perforated, allows joint compound to penetrate through the openings so that the mesh tape is sandwiched between an inner film of joint compound adhering to the drywall and an outer film overlying the mesh tape. The mesh tape reinforces the joint compound and substantially eliminates the problem of cracking along the outer edge of the flange. However, this type of cornerbead is subject to disadvantage because it is expensive to manufacture.